Operation of trucks and other heavy equipment requires an operator to frequently shift gears. Modern tractor trailer transmissions typically include 18 gear settings. Control of the transmission is accomplished by moving a shift tower through positions arranged in an H-shaped pattern. A selected gear ratio is engaged when the tower is pushed or pulled into a position at an end of a leg of the H-shaped pattern. A hydraulic switch located on the tower allows the driver to select whether the positions on the H-shaped pattern correspond to low, high, or overdrive combinations.
A large number of gear selections allows a driver to operate the vehicle efficiently, to navigate inclines and declines, and to quickly decelerate or accelerate in emergency situations. To make effective use of the gear selection, an operator must be able to rapidly move the tower among the gear selections and to easily access the switch.
During normal operation on the open road a driver may shift gears hundreds of times in an hour. In heavy traffic, or in stop and go city driving, even more frequent shifting may be required. Operators often operate vehicles up to ten hours per day and upwards of 250 days per year. Thus, an operator may need to shift gears half a million times or more per year.
Shifting gears places significant, repetitive strain on muscles and tendons that control motions of the driver's wrist and elbow that can lead to injury. Operating a manual transmission requires that the driver apply force to the tower to assure that the transmission is firmly engaged in the desired gear. Moreover, gear shifting involves moving the tower through the same H-shaped pattern over and over again. Furthermore, the location and angle of the tower may not be ideal for a particular driver, given the location of the seat, the height and arm-length of the driver and the fact that the driver must also work foot pedals simultaneously with the gear shifter. As a result, drivers sometimes suffer from repetitive motion injuries to the wrist and elbow. These injuries include wrist flexor tendinopathy (golfer's elbow), which is an inflammation of the tendon of the wrist flexor muscle.
Orthopedists recommend that people who perform tasks that require the same motion repeatedly make an effort to rest the limb performing the repetitive task. For truck drivers, gear shifting is a constant activity and, unless the driver pulls off the road (which costs time and money) there is no opportunity to rest. To the extent a repetitive motion cannot be avoided, orthopedists recommend adjustment of the range of motion to reduce overuse of particular tendons.
Located near the top of the tower are switches that allow the driver to select low, high, and overdrive gear combinations and/or to select intermediate (split) gears. The switches are positioned near enough to the grip so that the driver can quickly change from low to high to overdrive while at the same time moving the tower. Operation of the switches must be done simultaneously with motion of the tower to smoothly transition between gears.
The bottom end of the tower engages the transmission located along the center-line of the truck below the floor of the cab. The distance the top of the tower must travel through the H-shaped pattern discussed above in order to engage gears depends on the design of the transmission and on the height of the tower. These dimensions are fixed for a particular truck. Moreover, trucks made by different manufacturers, and even trucks made by the same manufacturer, but with different transmissions, may have different distances through which the tower most be moved. In many cases, drivers working for companies that employ a fleet of trucks (e.g., Federal Express, United Parcel Service, etc.) may be assigned to a variety of trucks. As a result, the distance a driver must reach through to shift gears may change from trip to trip, depending on which truck he or she is assigned.
Trucks are usually provided with seat adjustments that allow the driver to position the height of the seat, the distance of the seat from the controls, and the angle of the seat. Likewise, many trucks have adjustable steering columns and fuel, brake, and clutch pedals to allow the driver to comfortably operate the vehicle. The gear shift tower lacks such ergonomic adjustments. Instead, the location of the gear shift and the distance through which the tower must be pushed or pulled is determined by the overall shape of the cab and the design of the transmission. Likewise, the angle of the tower grip with respect to the driver's wrist is fixed by the design of the vehicle and cannot be adjusted to accommodate the shape of a drivers hand or the driver's range of comfortable motion. As a result, drivers are unable to make ergonomic adjustments to the tower grip. In addition, the grip on the tower does not provide a variety of surfaces to allow the driver to vary the position of his or her wrist, elbow, and shoulder when shifting gears.
The problem of injuries and strain resulting from poor ergonomics of gear shift tower is more acute for drivers with less upper body strength, and with drivers that suffer from arthritis, osteoporosis, and loss of muscle due to aging. In the United States, the average age of truck drivers is increasing as fewer younger drivers enter the business. Also, there is an increasing number of women, who general have less upper body strength than men. Thus, there are increasing numbers of drivers with reduced upper body strength that can benefit from an adjustment that can make the gear shift tower easier to operate.